The purpose of this study is to increase the performance and suppress the fluctuations in scaled MOSFETs by utilizing the quantum effects. We fabricated ultra-narrow channel MOSFETS and observed the threshold voltage increase by the quantum effect, which is confirmed by the numerical simulations. In the experiments, in order to confine electrons not only vertically but also laterally, extremely narrow silicon channels are fabricated by the electron beam lithography and dry etching technique. The channel width is varied from 2nm to 100nm. The channel width is very uniform and its distribution is less than 2nm. The dependences on channel orientation and polarity of carriers are also investigated. The threshold voltane rapidly increases when the channel width is less than 10 nm both in NMOS and PMOS.In order to clarify the origin of these phenomena, the Schrodinger equations are solved by the finite element method and the electron states in narrow channels are obtained. The results show that the threshold voltage increase is caused by the quantum confinement. We refer to this effect as the quantum mechanical narrow channel effect. This effect can be utilized to suppress the fluctuations and control the threshold voltage.